rebase-interactive.c: silence format-zero-length warnings

rebase-interactive.c: silence format-zero-length warnings

Fixes the following warnings:

rebase-interactive.c: In function ‘edit_todo_list’:
rebase-interactive.c:137:38: warning: zero-length gnu_printf format string [-Wformat-zero-length]
    write_file(rebase_path_dropped(), "");
rebase-interactive.c:144:37: warning: zero-length gnu_printf format string [-Wformat-zero-length]
   write_file(rebase_path_dropped(), "");

Signed-off-by: Ralf Thielow <redacted>
Signed-off-by: Junio C Hamano <redacted>